@Natalia ***
He should expect a fine of 500THB a day, with a maximum fine of 20K THB. If he is past 90 days, then it's a one year blacklist.
HOWEVER... I highly recommend for him to go to the local immigration office and apply for a 60 day extension of stay based on unable to travel due to COVID-19. This extension of stay is valid for 60 days, and the cost will be the overstay fine + 1,900 THB.